Simple unbridged metallocenes, such as bis(cyclopentadienyl)zirconium dichloride, are relatively inexpensive to synthesize compared with metallocenes having more complicated ligand structures, such as bridged metallocenes. However, when used to copolymerize olefins, unbridged metallocenes tend to incorporate comonomer poorly and chain terminate following comonomer insertion, producing olefin polymers with unacceptably low short chain branch frequency and molecular weight. In order to take advantage of their low cost, it would therefore be desirable to boost the ability of simple, unbridged metallocenes to make high molecular weight polyolefins having increased short chain branch frequency at a given level of comonomer.
EP 0 582 480 A2 describes an olefin polymerization catalyst comprising (A) an organoaluminum oxy-compound, (B) a transition metal compound of a Group IVB metal containing one or more ligands having a cyclopentadienyl skeleton, and (C) a hydrogenated organoaluminum compound of the formula H.sub.n AlR.sub.3-n, wherein R is an alkyl, cycloalkyl or aryl group and n is 1 or 2. The catalyst may also contain a carrier.
EP 0 406 912 B1 and EP 0 287 666 B1 relate to catalyst compositions comprising (A) a transition metal compound of the formula R.sup.1 R.sup.2 R.sup.3 R.sup.4 Me, wherein R.sup.1 is a cycloalkadienyl group, R.sup.2, R.sup.3, and R.sup.4 are cycloalkadienyl groups or other moieties, and Me is zirconium, titanium or hafnium, (B) an aluminoxane, and (C) an organoaluminum compound having a hydrocarbon group other than an n-alkyl group, which catalyst compositions are optionally supported. Polymers produced using such catalysts are reported to have narrow molecular weight distributions and narrow compositional distributions.
Similarly, U.S. Pat. No. 5,122,491 describes olefin polymerization in the presence of a catalyst composition prepared from (A) a Group IVB transition metal compound, or (A') a Group IVB transition metal compound supported on a fine-particle carrier, (B) an aluminoxane, and (C) an organoaluminum compound of the formula R.sup.1.sub.m Al(OR.sup.2).sub.3-m or R.sup.3.sub.m Al(OSiR.sup.4.sub.3).sub.3-m, wherein R.sup.1, R.sup.2, and R.sup.3 are hydrocarbon radicals, R.sup.4 is a hydrocarbon, alkoxy, or aryloxy, and m and n are positive numbers from 0 to 3.
Applicants have discovered that the short chain branch frequencies of olefin polymers produced using simple unbridged metallocenes are increased at a given level of comonomer when the unbridged metallocenes are used in catalyst compositions that also contain aluminoxane-impregnated supports made by contacting an aluminoxane with an inert carrier material and heating to a temperature of at least about 80.degree. C., bulky aluminum alkyls of the formula: EQU AlR.sup.1.sub.x R.sup.2.sub.(3-x)
wherein R.sup.1 is a saturated or unsaturated hydrocarbyl group having 1 to 12 carbon atoms; x is an integer from 0 to 2; R.sup.2 is a hydrocarbyl group of the formula --(CH.sub.2).sub.y --R.sup.3, wherein y is an integer from 1 to 8; and R.sup.3 is a saturated or unsaturated hydrocarbyl group having 3 to 12 carbon atoms containing at least one ring of at least 3 carbon atoms, and methylaluminoxane. When the mole ratio of aluminum from the bulky aluminum alkyl to metal from the metallocene is about 500, the short chain branch frequency of an olefin polymer made with such a catalyst is often at least about double that of a similar olefin polymer made under identical polymerization conditions using the same olefin monomers in the same molar ratio in the presence of a similar catalyst composition containing the same metallocene, but not the combination of the metallocene with an aluminoxane-impregnated support as defined above, a bulky aluminum alkyl as defined above, and methylaluminoxane.